The present application relates to a heat-resistant insulating layer-provided separator and to a non-aqueous electrolyte secondary battery. In more detail, the present application relates to a heat-resistant insulating layer-provided separator, which includes a polyolefin layer and a heat-resistant insulating layer obtained by containing a prescribed proportion of an oxidation-resistant ceramic particle in a heat-resistant resin, and to a non-aqueous electrolyte secondary battery using the same.
In recent years, following diffusion of portable information electronic devices such as mobile phones, video cameras and laptop personal computers, it is devised to realize high performance, downsizing and weight saving of these devices.
As a power source for these devices, disposable primary batteries and repeatedly usable secondary batteries are used. From the viewpoint of favorable comprehensive balance among high performance, downsizing, weight saving, economy and the like, secondary batteries, in particular, lithium ion secondary batteries have been increasingly demanded.
Also, in these devices, it is further advanced to attain higher performance and more downsizing. It is also demanded to realize a high energy density for lithium ion secondary batteries.
Following this, it is advanced to attain a high capacity of lithium ion secondary batteries by not only improvement and modification of electrode materials but improvement of a battery structure.
As one of methods for attaining a high capacity, an increase of a use charge upper limit voltage (open circuit voltage in a completely charged state per pair of a positive electrode and a negative electrode) (hereinafter abbreviated as “use charge upper limit voltage”) is watched.
In existing lithium ion secondary batteries, lithium cobalt oxide is used as a positive electrode, a carbon material is used as a negative electrode, and the use charge upper limit voltage is set up at from 4.1 to 4.2 V. In the lithium ion secondary batteries in which the use charge upper limit voltage is set up in such a way, in positive electrode active materials to be used for the positive electrode, such as lithium cobalt oxide, the capacity is utilized only in a proportion of from about 50 to 60% relative to a theoretical capacity.
For that reason, it is theoretically possible to utilize the residual capacity by further increasing the use charge upper limit voltage.
Actually, it is known that it is possible to attain a high energy density by regulating the use charge upper limit voltage at 4.25 V or more (see WO 03/019713).
Also, in lithium ion secondary batteries, as high capacity thereof becomes high, the energy density increases, too. Therefore, in the case where large energy is released in a superheating test or an internal short circuit test, a demand for enhancement in reliability is extremely large.
For that reason, lithium ion secondary batteries in which high reliability to such a test and high capacity are compatible with each other are earnestly demanded.
General lithium ion secondary batteries include a positive electrode containing a lithium composite oxide, a negative electrode containing a material capable of occluding and releasing a lithium ion, a separator lying between the positive electrode and the negative electrode and a non-aqueous electrolytic solution, in which the positive electrode and the negative electrode are wound via the separator, thereby configuring a group of columnar electrodes.
The separator has a function to electrically insulate the positive electrode and the negative electrode from each other and a function to hold the non-aqueous electrolytic solution. As such a separator of the lithium ion secondary battery, it is general to use a polyolefin microporous film.
This is because it is considered that in order to prevent the generation of a combustible gas or the rupture or ignition of the battery to be caused due to an abrupt increase of the battery temperature from occurring at the time when an abnormal large current flows due to induction of external short circuit or minute internal short circuit of the lithium ion secondary battery, the polyolefin microporous film shrinks or melts by its heat, thereby plugging pores to exhibit a function to shut down the ion permeation (shutdown function), too.
However, even if the shutdown function works, when the temperature of the lithium ion secondary battery further increases, there is a problem of the generation of so-called “meltdown” that the separator melts or thermally shrinks, whereby the positive electrode and the negative electrode cause short circuit on a large scale.
Also, for the purpose of enhancing the shutdown function, when heat melting properties of the separator are increased, there is a problem that the meltdown temperature of the separator is lowered.
Then, for the purpose of enhancing both shutdown properties and resistance to meltdown, for example, there is proposed a separator composed of a substrate layer including a porous film and a layer including a heat-resistant nitrogen-containing aromatic polymer such as aromatic polyamides or polyimides and a ceramic powder (see Japanese Patent No. 3175730).